Generally, pipes are roughly classified into welding pipes and seamless pipes. An electric resistance welding pipe as one of the welding pipes is manufactured by curling a sheet by roll forming or the like, and then confronting each edge and welding. In the welding pipes, toughness and strength of weld are generally bad compared with those of a mother strip. In any case of using a pipe, it is a problem to insure toughness and strength of weld for each application.
For example, since a line pipe for transporting crude oil or natural gas is often laid in the cold latitudes, low temperature toughness is essentially concerned. Moreover, strength is importantly concerned with a casing for protecting a mining pipe in an oil well for mining crude oil.
Typically, a hot-rolled sheet (strip) to be a mother strip of an electric resistance welding pipe is designed in composition or subjected to heat treatment in consideration of properties of the mother strip after pipe manufacturing to secure properties of the mother strip such as toughness and strength.
However, since characterization of welded seam is largely dependent on an electric resistance welding method compared with the composition design or heat treatment of the mother strip, improvement in welding technique is important.
As a cause of defective electric resistance welding, oxides called penetrators are given, which are generated on an edge of a welding sheet material. In many cases, the penetrators are not discharged from the edge along with melting steel during electric resistance welding and remained thereon, and the remained penetrators cause reduction in toughness, leading to insufficient strength.
Thus, to remove the penetrators from weld, earnest investigation has been made so far on an active discharging technique of melting steel from lateral edges of a strip to be welded in the weld. For example, JP-A-57-031485 or JP-A-63-317212 describes an example of investigation on a shape of each lateral edge of the strip. In the example, it is intended that the lateral edge of the strip, which is typically formed in an approximately rectangular shape by slitting or edge shaving, is processed in shape before electric resistance welding is performed such that the processed shape of the lateral edge improves discharge of melting steel during welding. This is summarized as follows.
That is, a basic manufacturing line of the electric resistance welding pipe is as shown in FIG. 1. The manufacturing line of the electric resistance welding pipe has a configuration where a strip 10 is uncoiled from an uncoiler 1; then the strip is reformed to be flat by a leveler 2; then the strip 10 is gradually rounded by a roll forming machine 4; then right and left, two lateral edges of the rounded strip 10 are welded by electric resistance welding using an electric resistance welder including an induction heating section 5 and a squeeze roll (electric resistance welding section) 6 so that the strip is formed into a pipe 30; then a weld bead portion of the pipe 30 is cut by a bead cutter 7; then the pipe 30 after cutting is adjusted in outer diameter by a sizer 8; and then the pipe is cut out into a predetermined length by a pipe cutter 9. The roll forming machine 4 has a predetermined number of finpass forming stands 3 that constrain a strip edge that has been rounded in a last stage to shape it into an approximate round, the stand 3 including a first stand 3a and a second stand 3b herein.
In the technique described in JP-A-57-031485, as shown in FIG. 5A showing a cross section diagram and FIG. 5B showing a partial detail diagram thereof, in the finpass forming first-stand 3a, a part of a lateral edge of the strip 10 formed into the pipe shape is contacted to a fin of a finpass hole-shape roll so as to shape tapering on a lateral edge to be an edge at an inner surface side of the pipe as shown in FIG. 5C, and as shown in FIG. 5D showing a cross section diagram and FIG. 5E showing a partial detail diagram thereof, in the finpass forming second-stand 3b, another part of the lateral edge of the strip 10 is contacted to the fin so as to shape tapering on a lateral edge to be an edge at an outer surface side of the pipe as shown in FIG. 5F, and thus an X-groove is formed. An angle of a fin of each of the finpass forming first-stand 3a and the finpass forming second-stand 3b is a typical one angle.
In the technique described in JP-A-63-317212, as shown in FIG. 6A showing a cross section diagram, an edger roll 11 is arranged at an upstream side of a finpass forming stand, and the edger roll 11 is used to reduce a lateral edge of the strip 10 formed into the pipe shape so as to shape tapering on the lateral edge of the strip 10 as a whole as shown in FIG. 6B, and as shown in FIG. 6C showing a cross section diagram and FIG. 6D showing a partial detail diagram thereof, in a finpass forming stand 3, a part of the lateral edge of the strip 10 is contacted to a fin of a finpass hole-shape roll, thereby a lateral edge to be an edge at an outer surface side of the pipe is shaped to be a vertical surface as shown in FIG. 6E. JP-A-2001-170779, JP-A-2001-259733 and JP-A-2003-164909 describe examples of investigating a shape of a strip edge. That is, it is intended that a strip edge, which is typically formed in an approximately rectangular shape by slitting or edge shaving, is tapered before roll forming such that the processed edge shape improves discharge of melting steel during welding.
However, we investigated the method described in JP-A-57-031485 and, as a result, found that even if the amount of upset in finpass forming was greatly changed, it was significantly difficult to contact only a part of the lateral edge of the strip 10 to the fin of the finpass hole-shape roll. This is because since the lateral edge of the strip 10 was slightly work-hardened in a previous forming process, the whole lateral edge of the strip is easily deformed along the fin so as to perfectly fill the fin portion, consequently a shape of the fin is printed to the lateral edge of the strip. As a result, the lateral edge of the strip 10 is not in a desired shape immediately before electric resistance welding is performed, and in an extreme case, the lateral edge is in a flat shape having a slope at only one side.
Moreover, we investigated the method described in JP-A-63-317212 and, as a result, confirmed the following. That is, to shape tapering to the whole lateral edge of the strip 10 using the edger roll 11 during roll forming (at the upstream side of the finpass forming stand), since the edger roll, of which the diameter is gradually increased from a pipe outer-surface side to a pipe inner-surface side, needs to be used for forming as described in JP-A-63-317212, a lateral edge to be an edge at the pipe inner-surface side is shaved by the edger roll, which may problematically induce pads called “whisker.” Furthermore, since large reaction force that opens the pipe-shaped strip 10 outward is exerted in a cross section direction of the strip 10 to be subjected to roll forming, pressure between the edger roll 11 and the lateral edge of the strip 10 is necessarily reduced. As a result, as in JP-A-57-031485, the strip is hardly work-hardened through reduction of the lateral edge by the edger roll, and even if the amount of upset is reduced in subsequent finpass forming, the strip substantially fills the fin portion, therefore it is difficult that the lateral edge of the strip 10 is shaped as described in JP-A-63-317212, consequently the tapering is completely eliminated, and the edge becomes flat.
It would therefore be advantageous to provide a method of manufacturing an electric resistance welding pipe, in which a lateral edge shape can be made into an appropriate shape immediately before electric resistance welding is performed, thereby melting steel is sufficiently discharged during electric resistance welding so that penetrators are securely removed, consequently an electric resistance welding pipe having excellent characterization of welded seam can be obtained.
As described before, in JP-A-57-031485 or JP-A-63-317212, a part of the lateral edge of the strip is pressed against the fin of the finpass hole-shape roll to shape the tapering to the lateral edge of the strip. However, according to the investigation, we understood that, even if the finpass hole-shape roll was not wholly filled with the strip in a circumferential direction, when the strip was loaded into the finpass hole-shape roll, the lateral edge was highly pressurized by the fin, so that the fin portion was perfectly filled with the lateral edge. That is, we understood that, when the strip was loaded into the finpass hole-shape roll, a lateral edge portion of the strip being contacted to the fin and a laterally central portion of the strip (portion of a bottom of the pipe-shaped strip) situated approximately 180 degrees opposite to the lateral edge portion were in a beam deflection condition, so that reaction force of the strip that acted to bend the cross section of the pipe-shaped strip into an arcuate shape was greatly exerted, consequently even if the strip did not fill the finpass hole-shape roll, large compression force was exerted on the lateral edge of the strip in the circumferential direction, as a result, the lateral edge of the strip was highly-pressurized by the fin, and consequently a shape of the fin was directly printed to the lateral edge of the strip.
Thus, we noticed a phenomenon that the lateral edge of the strip was highly-pressurized by the fin in the finpass forming, and conceived a method of shaping the predetermined tapering on the lateral edge of the strip by actively using the phenomenon. That is, we found that when the fin was shaped with two or more stages of tapering, even if the amount of upset in finpass forming was small, the lateral edge of the strip was able to be shaped with desired tapering, thereby the lateral edge of the strip was able to be shaped with appropriate tapering immediately before electric resistance welding was performed.
Moreover, JP-A-2003-164909 discloses various chamfer shapes that facilitate adjustment of confronting pressure. However, it does not make any description on a point of discharging penetrators along with melting steel, and a point of improving characterization of welded seam (particularly low temperature toughness) by such penetrator discharging. Therefore, it is completely unclear that which shape may improve the characterization of welded seam (particularly low temperature toughness) among the various chamfer shapes disclosed therein.